Put reuse Gitaly connections behind feature flag

It's an attempt to fix mysterious context canceled errors that mostly happen on production

There's an article, that mentions concurrency limits for a single GRPC connection, but haven't found how these limits can be configured in Go. When I tried investigating, I've come across this comment https://github.com/grpc/grpc-go/blob/v1.46.0/stream.go#L229 (also not sure whether it's our case). I think we can put the functionality behind a feature flag and observe the implications.

Let's wait until the command is executed and see whether it helps.

Related discussion: #559 (comment 934387869)

Supporting https://gitlab.com/groups/gitlab-org/-/epics/8014+